Light-emitting diode (LED) was launched to the market in 1965 when engineers used GaAs as a basis to develop the first LED device. The LED device emits red light with a small power and a luminous flux F≦0.01 lumen. The development of LEDs was pretty slow in the 70s and 80s. Even in the early 90s, the luminous flux of a fine green LED did not exceed 0.1˜0.3 lumen.
However, after the important invention of a Japanese professor, Shuji Nakamura, the LED development has seen acceleration. In the mid-90s, Nakamura proposed a new framework of nitride semiconductor for LED, whose substrate is based on Indium Nitride (InN) and Gallium Nitride (GaN). The unique feature of the semiconductor is that it comprises a large number of quantum wells in nanometer scales. The quantums are formed by adding oxygen as an activating agent during the synthetic process of the nitrides. Nakamura has summarized the process in one of his 1997 technical papers, Blue Laser (please refer to S. Nakamura, Blue laser, Springer Verlag, Berlin, 1997.).
Engineers in the Nichia company, which Nakamura has worked for previously, found a new application of blue LEDs, from which they developed a white-light semiconductor light. These Japanese engineers have developed the white-light LED based on the following known phenomena and frameworks. (1) The white light is generated according to the Newton Complementary Color Principle. (2) The two complementary colors, blue and yellow, are employed to generate uniform white light, which has been applied in the cathode ray tube CRT) of black-and-white television in the 40s. (3) The light-emitting surface of In—Ga—N heterojunction is spread with fluorescent powder with polymer adhesive; Bell company has previously employed a fluorescent powder layer to convert infrared emitted from a LED into visible light. (4) Properly choose the transparency and concentration of fluorescent powder layer so that the 20% first-order blue light of the light transmitting through the flourescent powder layer converting other blue light into yellow light radiation. (5) Use garnet containing yttrium aluminum and cerium as activating agent, i.e. (Y,Ce)3Al5O12, as the substrate of the yllow fluorescent powder, which was developed by a well-known Dutch scholar G. B.asse for special radiolgical and electronic instruments in 1965.
Consequently, white LEDs based on the aforementioned material have been widely used since 1996.
FIG. 1 illustrates the structure of a conventional warm-white LED, which is consistent with the invention of the Japanese engineer Schimzu. The aforementioned light source comprises the following components: a radiation heterojunction (P-N junction) 1; two conducting wires 2, 3; a heat-conducting base based on sapphire (Al2O3) or silicon carbide (SiC) 4; a reflection holder 5; a light converter formed as a polymer covering layer 6, in which is distributed with fluorescent powder 7; and a light shield formed as a ball or cylindrical lens 8, inside which is disposed with transparent polymer layers 9.
In fact, all white LEDs have a framework similar to the aforementioned one with only slight changes, and thus the aforementioned structure can be regarded as all-purpose.
Although the aforementioned framework has been widely applied, there exists some substantive drawbacks: Non-uniform distribution of fluorescent powder in polymer layer leads to non-uniform brightness and color of light emitted from LEDs. The radiating edges of the nitride heterojunction do not have a covering layer, leading to a large amount of blue radiation. Also, the color temperature of LEDs is very high T>8000K.
Many researchers have focused on solving the drawback of high color temperature, including U.S. Pat. No. 7,071,616 (refer to Schimizu Yetand U.S. Pat. No. 7,071,616). The patent discloses a LED with a low color temperature T≈3800˜6000K, which is achieved by using a fluorescent powder emitting an orange-yellow light with chromaticity coordinates: X>0.49 and Y>0.44. This LED can emit normal white light with a color temperature T≈4200˜4800 K. It characterized by the fact that its luminous intensity is very high, I>100, at 2θ=6°.
Although the aforementioned LED structure has many advantages, with regard to normal color temperature and high luminous intensity, the given scheme has a substantive drawback: its color temperature is different from that of an incandescent light source. The color temperature of an incandescent light source is 2850˜4000K. From the moment of inventing of incandescent lights 150 years ago, people's eyes have already used to the color temperature of this warm-white light source. The large amount of the red light in an incandescent light renders the surrounding objects with a natural color tone. In particular, human faces are comfortably illuminated, but they are somewhat un-natural under a fluorescent light. The drawback remains to be improved.